Distillazione degli idrocarburi

OIL, PETROL AND OCTANE NUMBER

Oil, or petroleum, is a complex mixture of solid, liquid, and gaseous hydrocarons, it also contains traces of organic components, oxygen, sulphur, and nitrogen. At room temperature oil is a thick, flamable oily, yellow to black substance.

Composition

Naturally occurring oil generally contains three hydrocarbons: alkanes, cycloalkanes and arenes. Other thypes of hydrocarbons like alkenes, alkynes and polyenes are more rare and only present in small quantities.

Oil also contains a wide variety of sulphurous compounds that have  a negative effect on the quality of the oil, since they cause unpleasant odours, corrosion, and pollution.  Nitrogen compounds, on the other hand, usually make up no more than 0.5 % of oil; oxygen compounds (mainly organic acids) do not usually exceed the 0.8 %.

Formation

Oil forms through long processes involving the decomposition of biological materials that settled to lake, sea, and ocean bottoms.

The factors involved in the formation of oil are: an anaerobic environment ( either in the sea or in a salty lake) where anaerobic bacteria  can degrade the materials; enzymes that can start the deamination (the removal of the amine group) of  peptides, and the decarboxylation ( removal of the carboxyl group) of fatty acids.

Mineral catalysts formed at elevated pressures also contribute to the process, and through slow reactions, after long periods of time (between 1 million and 600 milion years), at high pressures (up to 900 atm) and tempertures (on the order of 150 °C), oil is formed. 

Oil forms in droplets in sedimentary rock, it would be impossible to extract it from that type of rock, but oil then migrates into reservoirs that are like geological traps: a alyer of porous rocks situated between two layers of impermeable rocks .Gaseous  hydrocarbonsare trapped  there, toghether with oil, and water. the sediment is layered from top to bottom depending on density.

Oil refining
Once crude oil has been extracted, it is refined, i.e. fractioned into components with different density levels. The process is divided into 4 main phases:
- Fractional distillation is carried out in columns that have a  number of trays at different levels. The  components condense at different levels depending on their boiling temperature. After a first topping ( atmospheric distillation)  witw teperatures reaching 350 °C, there is a second, vacuum distillation with temperatures up to 400 °C. The components are gasses with a low molecular weight (methane, kerosene, diesel, heavy residue).
The heavy residue undergoes a second vacuum distillation process, resulting in kerosene, diesel, lubricant oil and a residual (bitumen or asphalt).

- Cracking process aimsat increasing the percentage of petrols, usually no more than 25% of the distillation products, by turning diesel and kerosene, that have 10 to 16  atoms of carbon, into lighter elements. At 450 °C and tranks to certain catalysts, catalytic cracking  produces short-chained or branched molecules (ethene and propene for production of plastic, LPG as a household heating fuell).

- Catalytic reforming process has been used sinse 1949, and it has improved petrol quality, as it increases their antiknock properties.

- Hydrodesulphurisation process allow to recover sulphur in oil ( due to the formation of H₂S and H₂SO₄ ), and it also improves petrol quality, since it prevents the formation of polluting compounds during combustion. 

Da Chimica e Vita  - Bargellini, Crippa, Nepgen, Mantelli